Shell small, equivalve, very inequilateral; anterior end very little developed, or atrophied; byssus short, thin, passing through a narrow opening between the anterior margins of the valves; umbos projecting and formed by the embryonic shell or prodissoconch, straight or slightly curved toward the anterior end (prosogyrate) ; umbonal cavity well marked; ligament internal, or only partly external, median or directed backwards; area linear, epidermical or absent; hinge with strong vertical crenulations; in front and behind this double border of crenulations there are marginal teeth or edges, which sometimes become obsolete; shell not nacreous, without a prismatic layer, sometimes polished, finely tubular. Animal like that of Limopsis, usually without an anterior adductor muscle. Viviparous. Carnivorous. Distribution: Cosmopolitan, from the littoral zone to 200 fathoms.

The philobryid shell is small (to 10 mm), solid, and obliquely oval to quadrangular. It is EQUIVALVE, compressed to moderately inflated, and in some species with a distinct an-teroventral byssal gape. The shell is INEQUILATERAL (umbones anterior), with an inflated posterior slope and reduced anterior end. The UMBONES are PROSO- or ORTHOGYRATE, and ARAGONITIC and two-layered, with a CROSSED LAMELLAR outer laver and a COMPLEX CROSSED LAMELLAR inner layer. TUBULES are present on the internal surface (except near the margin) and do not open onto the external surface. Exteriorly philobryids are covered by a thick, dehiscent, hirsute PERIOSTRACUM that extends beyond the shell margin. Sculp¬ture is finely radial, in some cases with commarginal striae. LUNULE and ESCUTCHEON are absent. Interiorly the shell is non-NACREOUS. The PALLIAL LINE is ENTIRE. The inner shell margins are smooth or denticulate. The HINGE PLATE is straight to weakly arched, in some cases narrow and reduced TAXODONT teeth (sometimes called PROViNCULAR-like) on ei¬ther side of a central RESILIFER; Philobrya is EDENTATE in adults and has a dreissenid-like myophoral SEPTUM under the umbo. The LIGAMENT is ALIVINCULAR (perhaps reduced DU-PLIVINCULAR) and AMPHI- or OPISTHODETIC, with a small internal portion (RESILIUM).
The animal is MONOMYARIAN (anterior ADDUCTOR MUSCLE absent) or extremely HET-EROMYARIAN (anterior adductor muscle smaller); the pedal retractor muscles are well developed, with the posterior ones larger. Pedal protractor muscles have not been reported. The MANTLE margins are not fused ventrally; SIPHONS are absent. HYPOBRANCHIAL GLANDS are present in the SUPRABRANCHIAL CHAMBER along the gill axis. The MANTLE
margins are nontentaculate, and have simple PALLIAL ("siphonal") EYES (cap-shaped, without lens) on the outer folds, covered by periostracum. The FOOT is digitiform, later¬ally compressed, small anteriorly, and with a large, thin posterior heel. It has a BYSSAL GROOVE; the adult is byssate.
The LABIAL PALPS are-small. The CTENIDIA are FILIBRANCH (ELEUTHERORHABDIC), HOMORHABDIC, of about equal size, with unusually short filaments that are not inserted into (or fused with) the distal oral groove of the palps (CATEGORY III association). CEPHALIC EYES (cup-shaped, with lens) are present in Philobrya munita Finlay, 1930, on each inner demibranch at the junction of the inner labial palp. Incurrent and excurrent water flows are posterior; there can also be an anterior incurrent. The lips are simple; the STOMACH is TYPE III. The MIDGUT is not coiled. The HINDGUT passes either ventral to or
through the ventricle of the heart, and leads to a freely hanging rectum. Philobryids are GONOCHORISTIC and viviparous, producing large, yolky eggs. Brooded larvae are attached to the byssus or gills during incubation. As a result, the gills have short, robust filaments and few interlamellar junctions, and the adult shell can have a large, sharply demarcated PRODISSOCONCH. The nervous system is not concentrated. STATOCYSTS and ABDOMINAL SENSE ORGANS have not been reported.
Philobryids are marine SUSPENSION FEEDERS, living INFAUNALLY or epibyssally, the latter nestling in algae, hydroids, or mussel beds. Efficient EPIFAUNAL crawling has been reported for one species.
The family Philobryidae is known since the Eocene, is represented by 6 living genera and ca. 20 species, and is predominantly distributed in the southern oceans, in warm seas from the intertidal to great depths (1,000 m). Some features of philobryids (e.g., ligament type, adult byssus) suggest that their evolution involves PEDOMORPHOSIS (perhaps NEOTENY).

Shell aragonitic, with outer crossed-lamellar and inner complex crossed-lamellar layers. Inequilateral, equivalve; outline eliptical to subquadrate, compressed. Periostracum thick, hirsute, dehiscent, forming marginal fringe. Umbones prominent, with large, sharply demarcated prodissoconch. Hinge plate narrow, edentate, or with obscure to well-developed teeth. Ligament opisthodetic, internal or partly external. Dimyarian; anterior adductor muscle reduced or evanescent, leading to monomyarian adult. Mantle lobes not fused, except for small posterior inhalant aperture. Outer fold of mantle with ocelli, without tentacles. Ctenidia filibranch, eleutherorhabdic. Hypobranchial gland present. Labial palps small. Foot digitiform. Adult byssate. Alimentary canal with type 3 stomach; style-sac conjoined to mid-gut; intestine passing through ventricle of heart. Some, possibly all, species incubatory. The placement of this family is uncertain. Previously associated with the limopsids and glycymerids on the basis of anatomy, but its shell microstructure and the ligament are closer to the arcoids. Widely distributed in shallow water, frequently nestling in algae, hydroids, or in mussel beds. The family is well represented in New Zealand and the Antarctic regions, and tropical Indo-Pacific caves. Recognized since the Eocene, at least six genera are known with approximately 20 species. One genus is present in the northeastern Pacific.